New Threats on the Horizon

As Observatree reviews its priority pests and diseases list, an important question is being asked: which emerging threats are both significant enough to monitor and suitable for citizen science surveillance? The 2025 review examined fifteen organisms from the UK Risk Register (UK Plant Health Risk Register) alongside three additional organisms of concern from the European Plant Protection Alert List (EPPO Alert List). 

For citizen scientists, the challenge is not simply finding high-risk pests and diseases for surveillance. The ideal target must also have clear, visible symptoms that can be recognised from ground level without specialist equipment or destructive sampling. Just as importantly, it should not be easily confused with common native species.

One of the most promising additions discussed during the review is Puccinia sparganioides, a rust fungus affecting ash trees. Although native to North America, the pathogen has a UK Risk Register score of 30 (out of 125) and could potentially weaken trees through repeated infections. It produces striking orange fruiting bodies on leaves, petioles and young shoots, making it relatively easy to spot (Figure 1).

Additionally, there are currently no other rust species affecting ash trees in the UK, reducing the risk of misidentification. For citizen scientists, this combination of a distinctive appearance and impact makes it a potential surveillance target.

Another organism under consideration is Cryphonectria carpinicola, a fungus associated with hornbeam decline. The pathogen has attracted increasing attention across Europe, particularly where it occurs alongside another fungus, Anthostoma decipiens. The disease results in stem cankers, dieback and ultimately death (Figure 2).  While the impact of C. carpinicola alone remains uncertain, evidence suggests disease severity may increase when both fungi are present. Defra has funded research, under Future Proofing Plant Health, to investigate this disease complex. However, whilst we await outcomes of this research, any suspicious symptoms should be reported to Tree Alert.

The review also assessed several root and stem decay fungi within the Heterobasidion group. Of particular interest is Heterobasidion irregulare, a serious pathogen of pines with a UK Risk Register score of 75, making it one of the highest-risk organisms assessed. The fungus is established in North America and parts of Europe and could potentially cause significant damage if introduced into the UK. The fungus causes root and butt rot and produces large fungal fruiting bodies (Figure 3). It colonizes the cambial layer and sapwood of its hosts, both in the root system and at or just above the root collar. However, despite its high-risk status, diagnosis in the field may be challenging as it may require invasive sampling, highlighting the balance that must be struck between risk and practicality when selecting citizen science targets.

The review also considered several wood-boring insects associated with imported ornamental plants and bonsai. These included the Japanese horntail (Urocerus japonicus) and the Nitobe horntail (Sirex nitobei) (Figure 4). While both species have moderate risk scores and could damage timber through larval feeding and fungal associations, they closely resemble native UK wood wasps. For volunteers surveying trees in the field, separating these species with confidence would be extremely difficult, making them poor candidates for routine monitoring programmes.

Another species examined was the fall cankerworm (Alsophila pometaria), a North American moth capable of causing repeated defoliation of broadleaved trees (Figure 5). During outbreaks, entire canopies can be stripped of foliage, and repeated attacks may contribute to tree decline or death. Although its impacts could be substantial, the review highlighted that identifying the insect at all life stages may be difficult. This again illustrates a key principle of citizen science surveillance: a damaging pest is only a good target if volunteers can reliably recognise it.

The global distribution of pests and pathogens is always changing and our annual review for Observatree helps us to monitor the situation and inform not only what we highlight to the Observatree volunteers, but also to others with an interest in tree health. We are not proposing to add any of these at this time to our priority list, but we will review the situation again next year. Nevertheless, if you think that you have seen any of these or any others on our existing list, please report them through TreeAlert.